1. Technical Field
The present invention relates generally to infrared imaging devices and, more particularly, to a method and apparatus for compensating for the nonuniformity of response of the detector elements of an infrared focal plane array.
2. Discussion
Infrared imaging devices convert invisible infrared energy into useful forms such as visible images. This is accomplished by first detecting infrared radiation which is emitted or reflected from objects in a field of view and then producing resultant data corresponding to the signal flux level of the infrared radiation that is detected. Generally, infrared imaging devices include an optical system for receiving the infrared radiation, detector elements, such as in a focal plane array, for producing a response corresponding to the signal flux level of the infrared radiation received through the optical system, an electronics unit for processing the response produced by the detector elements and generating a corresponding digital output, and may also include an output display unit, such as a cathode-ray tube (CRT), for displaying a resultant visible image, if desired.
The operation of an infrared imaging device may generally be described as follows: the optical system surveys a field of view and causes infrared radiation to be directed toward the detector elements, each of which correspond to discrete points, or pixels; each detector element then generates an analog response, such as a voltage or current, corresponding to the signal flux level of the infrared radiation received by that detector element; the analog response is then processed in the electronics unit which produces a digital signal output; and, finally, the output is converted into a useable form, such as a visible image generated by an output display unit. Typically, this procedure is continuously repeated for generating a series of useable data in near real-time, depending upon the imaging application desired to be performed.
However, through experience it has been learned that because each individual detector element in a focal plane array is not identical, each detector element may produce a different response to a given signal flux level of infrared radiation that it receives. That is, a detector element "A" and a detector element "B" may be exposed to the same signal flux level of infrared radiation, however, each detector element may generate a unique analog response. In addition, the converse may also be true. That is, a detector element "A" and a detector element "B" may be exposed to different signal flux levels of infrared radiation, however, each detector element may generate the same analog response. This phenomenon is well-known as "non-uniformity of response" or, simply, "non-uniformity." It has also been learned that the relationship between the response produced by a detector element in relation to various signal flux levels of infrared radiation is non-linear.
Consequently, infrared imaging devices must compensate for the variations between detector elements and correct them to an "absolute scale" in order to produce consistent and reliable image data.
A means to accomplish the necessary "non-uniformity compensation" so that accurate image data may be generated by the infrared imaging device has been to calibrate each detector element in the focal plane array with the electronics unit. In this manner, the electronics unit produces a "corrected" digital signal output which reflects a "normalized" value for the signal flux level of infrared radiation that is received by a given detector element. Typically, non-uniformity compensation has been accomplished by the use of a two-point or a four-point calibration method having analog gain and offset adjustments. However, this technique has presented the disadvantage of allowing the analog response of any particular detector element to be compensated for non-uniformity only over a narrow dynamic range of signal flux levels of infrared radiation. Therefore, previous non-uniformity compensation schemes have been inadequate where the field of view of the imaging device has contained a wide dynamic range of signal flux levels of infrared radiation, such as a "cold" sky and a "hot" terrain.
In light of the forgoing discussion, it is a principal object of the present invention to provide a method and apparatus for non-uniformity compensation for use in the focal plane array of an infrared imaging device capable of compensating for non-uniformity over a wide dynamic range of signal flux levels of infrared radiation. In addition, it is another object of the present invention to provide a focal plane array uniform response which can be calibrated as an absolute radiometer.